This invention relates to the formation of contact and intraocular lenses, and more particularly to lens manufacture with computer controlled machine tools.
For many years, contact lens base curves have been generated by lathing a plastic lens blank while it is grasped in a standard drawback collet. Although this system has proven to be satisfactory in the past and has been used to provide millions of base curves for contact lenses, there are a number of problems that arise from this operation. Even if the button diameter is held to a very tight and repeatable tolerance, interferometer studies show that the base curve is discernibly distorted by the grasp of the collet. The tighter the grasp of the collet, the more distortion. Distortion can be minimized by reducing the grasp of the collet. However, going too far in this direction can cause the button to slip and as a result, an expensive diamond tool can be ruined. As lathing equipment improves and more accurate base curves are produced by the lathing operation, less polishing is required and generally, better optics are produced (less distortion), if polishing is kept to a minimum.
In order to avoid the distortion produced by having the lens blank in a collet, a number of base curve blocking systems have been developed by a variety of manufacturers of contact lenses in which the blank is adhered to the block with a thick film of wax adhesive. Generally the contour of the lens produced from blocked base curves is more accurate and has less distortion than base curves generated directly in drawback collets.
However, the use of thick film wax adhesives to block lens blanks before cutting base curves is not conducive to cutting a series of lenses with constant center thickness. The wax is usually applied as a thick film onto the surface of a blocking member and the lens blank applied to this thick film with pressure which varies from blank to blank. Another type of prior at base curve blocking member had a cavity on the surface of the block that was filed with hot wax. As the wax cooled, it contracted and pulled the lens blank into the cavity deforming the blank and distorting its shape during cutting of the base curve. When the lens blank with base curve was removed from the surface of the blocking member, the force is released. The lens blank returns to its original shape which changes the shape of the base curve.
Several manufacturers produce a constant center thickness base curve (and this is always an additional operation) to eliminate the necessity of measuring center thickness (a difficult operation that is subject to error) before mounting the base curve blank on a block for front surface cutting.